Projection-type display devices have been known as devices that are used for displaying an image and in each of which a Fresnel lens screen is combined with a diffusion sheet (i.e., a diffusion layer). Unlike Cathode Ray Tube (CRT) display devices and Plasma Display Panel (PDP) display devices, projection-type display devices are non-light-emitting-type display devices. A projection-type display device includes: an illumination optical system that, as a projector, guides light from a light source into a predetermined direction; a light bulb that is irradiated by the light guided by the illumination optical system and forms an image by adjusting the amount of light according to an image signal; a projection optical system that enlarges and projects the image that has been formed by the light bulb onto a screen.
Projection-type display devices include rear-surface projection-type display devices in which the image light is projected from the rear surface of the screen as seen from the viewer and front-surface projection-type display devices in which the image light is projected from the front of the screen as seen from the viewer. A transmission-type screen, which is used in the rear-surface projection-type display devices among these two types of display devices, includes a Fresnel lens screen that inflects the image light from the projector toward the viewer side and an image display element that forms an image with the image light from the Fresnel lens screen and enlarges the image light by applying a divergent angle thereto.
Generally speaking, Fresnel lenses are manufactured so as to have a pitch that is finer than a projected pixel (for example, one tenth of the pixel). Thus, the dimension in the thickness direction is extremely small (the thickness including a prism portion is, for example, hundreds of micrometers). Thus, to hold a Fresnel lens, it is necessary to use a substrate having a thickness of approximately 1 millimeter to 5 millimeters. The substrate is often formed of a resin such as Poly Methyl MethAcrylate (PMMA), Methyl methacylate Styrene (MS), Methyl methacylate Butadiene Styrene (MBS) or Polycarbonate (PC), or formed of glass. Also, in many situations, a Fresnel lens is formed directly on a substrate by using a light-curing resin or the like. An element that is made up of a Fresnel lens and a substrate is called a Fresnel lens screen. Such Fresnel lenses can be classified into light-entering-surface-side Fresnel lenses (hereinafter, “entering-side Fresnel lenses”) in which the prisms are formed on the light-entering-surface side and light-exiting-surface-side Fresnel lenses (hereinafter, “exiting-side Fresnel lenses”) in which the prisms are formed on the light-exiting-surface side. In addition, the image display element is, for example, a lenticular screen and at least includes a light diffusing means and a substrate.
The image display element has irregularities on the surface thereof and has a refractive index distribution, a phase distribution, and a transmittance distribution on the inside thereof. Thus, when the image light from the projector is viewed through a transmission-type screen, the image light has a fluctuation that is larger than the wavelength of the light. As a result, innumerable speckles (glares), some of which are bright and some of which are dark, are perceived on the transmission-type screen in a disorderly manner. These bright and dark speckles are generally called “speckles” or “scintillations” and can be a cause of image degradations.
As a countermeasure for such speckles (i.e., scintillations), some methods have been proposed such as a method by which a screen is oscillated (see Patent Document 1) or a method by which diffusion layers included in a screen are positioned apart from each other (see Patent Document 2).    Patent Document 1: Japanese Patent Application Laid-open No. 2006-343663    Patent Document 2: Publication of Japanese Patent No. 3606862